1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of performing a stable sheet transfer operation.
2. Discussion of the Background
A typical background sheet transferring apparatus used in an image forming apparatus, such as a laser printer, a plain paper copying machine, a facsimile machine, etc., is illustrated in FIG. 1. The background sheet transferring apparatus of FIG. 1 has a sheet passage for a recording sheet traveling from a sheet container 1 through a photoconductive member 12. In FIG. 1, a stack of recording sheets 2 stacked in the sheet container 1 are positioned such that leading edges of the recording sheets 2 are neatly aligned at an initial position A. When a sheet transfer operation is started, a sheet feed signal is turned on in an electrical control system (not shown) and is transmitted to the background sheet transferring apparatus. With the sheet feed signal, a pick-up roller 3 is lowered and is rotated so as to move the recording sheets 2 towards a position B where a sheet separation mechanism is provided. The sheet separation mechanism, namely, a friction reverse roller system includes a feed roller 4 for being rotated to limit and to move one recording sheet 2 forward and a reverse roller 5 for being rotated to move back the accompanying recording sheets 2. The feed roller 4 and the reverse roller 5 are driven at the same time the pick-up roller 3 is driven so that a recording sheet 2 is separated and is transferred forward. In this example, the feed roller 4, the reverse roller 5, and the pick-up roller 3 are driven with a motor (not shown).
After being separated at the position B by the friction reverse roller system, the recording sheet 2 is moved such that the leading edge of the recording sheet 2 reaches a photo sensor 6 located at a position C. Then, the pick-up roller 3 is lifted and is stopped to be driven so that the pick-up roller 3 loses a sheet transfer power for moving the recording sheet 2. After that, the recording sheet 2 is further moved to a transfer roller 7 located at a position E by a sheet transfer power of the feed roller 4. The feed roller 4 is stopped to be driven in a time period t1 (see FIG. 2) after having been driven so that the leading edge of the recording sheet 2 is moved to a position F downstream from the position E. After the feed roller 4 is stopped to be driven, the recording sheet 2 is further transferred by the transfer roller 7. The leading edge of the recording sheet 2 is then brought to pass a photo sensor 8 located at a position H and then to reach a position I when the trailing edge of the recording sheet 2 is brought away from the sheet separation mechanism. After that, the leading edge of the recording sheet 2 is further moved to a transfer roller 9 located at a position Exe2x80x2. In the above operations, the transfer rollers 7 and 9 are driven with a transfer roller driving motor (not shown). The recording sheet 2 is then transferred to a photo sensor 10 (referred to as a registration sensor 10) located at a position J and to a registration roller 11 located at a position K. Further, the recording sheet 2 is transferred to an image transfer section located at a position L and which is composed of the photoconductive member 12 and an image transfer roller 13.
FIG. 2 is a convenient graph with respect to a sheet transferring performance of the background sheet transferring apparatus, which is composed of a performance characteristic graph 1 to a time chart 1. The performance characteristic graph 1 demonstrates a characteristic of a sheet transfer operation of the background sheet transferring apparatus by showing successive positions of leading and trailing edges of a recording sheet in the sheet passage in response to a time parameter. The time chart 1 shows the sheet feed signal and the subsequent actions of the various components in connection with the movement of the recording sheets shown in the performance characteristic graph 1. In the performance characteristic graph 1, the vertical axis represents a distance from the initial position A to a position after the position K and the horizontal axis represents time. In the performance characteristic graph 1, with a time parameter, solid lines represent actual positions of the leading edge of a recording sheet 2 and thick broken lines represent actual positions of the trailing edge of the recording sheet 2. Thin two-dotted chain lines represent calculated positions of the leading edge of the recording sheet 2 without consideration of slippage of the recording sheets 2 relative to the rollers and wearing of the rollers. Thin broken lines represent calculated positions of the trailing edge of the recording sheet 2 without consideration of slippage of the recording sheets 2 relative to the rollers and wearing of the rollers. In this example, the recording sheet 2 has a letter size and is transferred in a direction of a short edge having a length of 216 mm.
In a time period t2 after the leading edge of the recording sheet 2 is brought to reach the registration sensor 10 at the position J, the transfer roller driving motor is stopped so that the transfer rollers 7 and 9 lose sheet transfer powers for moving the recording sheet 2. The time period t2 is determined so that the leading edge of the recording sheet 2 is brought to reach the registration roller 11. At this time, the registration roller 11 is not driven. With this determination of the time period t2, a skew correction is conducted. That is, the leading edge of the recording sheet 2 is brought to collide against the registration roller 11 so that the recording sheet 2 makes a slack before the registration roller 11 which corrects a skew if it exists. In this example, the time period t2 is set to 37.5 ms.
After that, the transfer roller driving motor is driven at the same time the registration roller 11 is driven so that the rotations of the transfer rollers 7 and 9 are restarted. Consequently, the recording sheet 2 is further transferred to the image transfer section so that an image formed on the photoconductive member 12 is transferred onto the recording sheet 2. The registration roller 11 is configured to turn on in a time period t3 after the photo sensor 8 at the position H is turned on. In this example, the time period t3 is set to 400 ms. With this time period t3, the movement of the recording sheet 2 is timed in synchronism with the rotation of the photoconductive member 12 so that the position of the image on the photoconductive member 12 matches the position of the recording sheet 2.
In the performance characteristic graph 1 of FIG. 2, distances of the various positions with reference to the initial position A are set as follows:
28 mm between the positions A and B,
38 mm between the positions A and C,
123.4 mm between the positions A and E,
133.4 mm between the positions A and F,
231.9 mm between the positions A and H,
244 mm between the positions A and I,
344 mm between the positions A and J,
359 mm between the positions A and K, and
216 mm between the positions B and I.
With the arrangement above, the following time periods t11-t16 are needed:
979.75 ms for the time period t11 in which the transfer roller driving motor is driven in synchronism with a rise time of the sheet feed signal;
1048.5 ms for the time period t12 from a rise time of the sheet feed signal to a time the registration roller 11 is turned on;
826.09 ms for the time period t13 from a rise time to the next rise time of the registration roller 11;
755 ms for the time period t14 between calculated times the leading edges of a recording sheet and the next recording sheet are forwarded by the registration roller 11;
252.5 ms for the time period t15 between calculated times the trailing edges of a recording sheet and the next recording sheet are forwarded by the registration roller 11; and
322.82 ms for the time period t16 between a rise time to a fall time of the registration sensor 10.
In addition, the time period t1 represents a time the feed roller 4 is being driven, the time period t2 represents a time from a rise time of the registration sensor 10 to a time the transfer roller driving motor is stopped, the time period t3 represents a time from a rise time of the photo sensor 8 to a time the registration roller 11 is driven, and the time period t4 represents a time from a fall time of the registration sensor 10 to a time the registration roller 11 is stopped.
In the above-described background sheet transferring apparatus, the transfer rollers are apt to lose the sheet transfer powers and the diameters due to wear over time and has a consequent tendency to increasingly cause an excess slippage against the recording sheet 2. This leads to a reduction of the sheet transfer linear speed and adversely affects a printing productivity. More specifically, in the sheet transfer process, the recording sheet 2 is transferred forward while being slipped against the rollers due to a given load such as a load from the reverse roller 5 in the sheet separation mechanism, a load from another recording sheet in close contact, or the like. Largeness of the load depends on the nature of the recording sheet 2, such as a size of the sheet, the surface of the sheet, etc. That is, there is a tendency that the recording sheet 2 suffering a small load causes a small slippage and the recording sheet 2 suffering a large load causes a large slippage. In addition, the recording sheet 2 increasingly causes such slippage with time due to a reduction of the sheet transfer power caused by the following phenomena. This is, the surface of the recording sheet 2 is changed by deposition of a paper dust or wear. Also, the transfer rollers have a friction coefficient xcexc which is reduced due to variations of rubber material over time. Furthermore, the reduction of the roller diameters due to wear with time causes another problematic reduction of the sheet transfer linear speed.
FIGS. 3A and 3B show various data associated with the performance of the background sheet transferring apparatus that has the sheet transfer linear speed of 400 mm/s. The data includes a ratio of a sheet slippage, a reduction of a roller diameter, a reduction of the sheet transfer linear speed, and an actual sheet transfer linear speed performed in each part of the sheet passage of the background sheet transferring apparatus. FIGS. 3A and 3B may be read as one data table having columns AA, BB, CC, DD, EE, FF, and GG.
In FIGS. 3A and 3B, the sheet passage is divided into the following passage parts, which are indicated in a column AA of FIGS. 3A and 3B:
A-B represents a passage part between the positions A and B, that is, from the initial position A to the sheet separation mechanism;
B-E represents a passage part between the positions B and E, that is, from the sheet separation mechanism to the transfer roller 7;
E-F represents a passage part between the positions E and F, that is, from the transfer roller 7 to the position F to which the leading edge of the recording sheet 2 is moved when the feed roller 7 is turned off;
F-H represents a passage part between the positions F and H, that is, from the position F to the photo sensor 8;
H-I represents a passage part between the positions H and I, that is, from the photo sensor 8 to the position I to which the leading edge of the recording sheet 2 is moved when the trailing edge of the recording sheet 2 is brought away from the sheet separation mechanism;
I-J represents a passage part between the positions I and J, that is, from the position I to the registration sensor 10;
J-K represents a passage part between the positions J and K, that is, from the registration sensor 10 to the registration roller 11; and
K-L represents a passage part between the positions K and L, that is, from the registration roller 11 to the image transfer section.
The components particularly activated and essential in the sheet transfer operations in each of the above-mentioned passage parts of column AA of FIGS. 3A and 3B are as follows:
A-B; the pick-up roller 3,
B-E; the pick-up roller 3 and the feed roller 4,
E-F; the feed roller 4 and the transfer roller 7,
F-H; the transfer roller 7,
H-I; the transfer roller 7,
I-J; the transfer rollers 7 and 9,
J-K; the transfer rollers 7 and 9, and
K-L; the registration roller 11 and the transfer rollers 7 and 9.
Load factors generated as a reverse force against the forward force of the sheet transfer operations in each of the above-mentioned passage parts of column AA of FIGS. 3A and 3B are as follows:
A-B; a close contact power between sheets by friction,
B-E; a close contact power between sheets by friction and a repulsive force from the reverse roller 5,
E-F; a close contact power between sheets by friction and a repulsive force from the reverse roller 5,
F-H; a repulsive force from the reverse roller 5,
H-I; a repulsive force from the reverse roller 5,
I-J; no particular load factor,
J-K; no particular load factor, and
K-L; no particular load factor.
In FIG. 3A, a column BB indicates a distance of each passage part and a column CC indicates an accumulated distance from the initial position A to the end of each passage part. A column DD is a ratio of a sheet slippage expressed as a percent and is divided into an initial condition DD1 and an after-predetermined-time-use condition DD2. Each of DD1 and DD2 is divided into two cases; MIN indicating a sheet slippage ratio under a minimum load and MAX indicating a sheet slippage ratio under a maximum load. A column EE indicates a diameter of the roller associated with the sheet transfer operations in each passage part. The column EE is divided into EE1-EE3: EE1 is an initial diameter; EE2 is a radial reduction amount expressed in a percent due to the wear after a relatively long time use, and E3 is an amount of reduction in the sheet transfer linear speed expressed in a percent due to the reduction of the roller diameter. In FIG. 3B, a column FF indicates an amount of a total reduction in the sheet transfer linear speed expressed in a percent, in which wear of the reverse roller 5 is taken into consideration. The column FF is divided into an initial condition FF1 and an after-predetermined-time-use condition FF2. Each of FF1 and FF2 is divided into two cases; MIN indicating a total reduction in the sheet transfer linear speed expressed in a percent under a minimum load and MAX indicating a total reduction in the sheet transfer linear speed expressed in a percent under a maximum load. A column GG indicates an actual sheet transfer linear speed. The column GG is divided into an initial condition GG1 and an after-predetermined-time-use condition GG2. Each of GG1 and GG2 is divided into two cases; MIN indicating the actual sheet transfer linear speed under a minimum load and MAX indicating the actual sheet transfer linear speed under a maximum load.
The data of the actual sheet transfer linear speed under the initial condition GG1 is referred to as GG1-MIN in the case the minimum load is provided and as GG1-MAX in the case the maximum load is provided. Likewise, the data of the actual sheet transfer linear speed under the after-predetermined-time-use condition GG1 is referred to as GG2-MIN in the case the minimum load is provided and as GG2-MAX in the case the maximum load is provided. For example, the solid lines and thick broken lines shown in the performance characteristic graph 1 of FIG. 2 are based on GG2-MAX.
In a similar manner, FIG. 4 demonstrates a linear speed graph expressing cases Z1, Z2, Z3, and Z4 based on GG1-MIN, GG1-MAX, GG2-MIN, and GG2-MAX, respectively, of FIGS. 3A and 3B. In FIG. 4, sheet transfer cycles from a recording sheet 2 to the next recording sheet 2 at the registration roller 11 in a continuous sheet feeding mode in the cases Z1, Z2, Z3, and Z4 are referred to as Z1a, Z2a, Z3a, and Z4a, respectively. Also, time differences from the trailing edge of a recording sheet 2 to the leading edge of the next recording sheet 2 at the registration roller 11 in the continuous sheet feeding mode in the cases Z1, Z2, Z3, and Z4 are referred to as Z1b, Z2b, Z3b, and Z4b, respectively.
Based on the above-mentioned sheet transfer cycles Z1a-Z4a, corresponding copy speeds of the image forming apparatus employing the sheet transferring apparatus are calculated in the following manner. In the case Z1, the sheet transfer cycle Z1a is 784.14 ms per a sheet and therefore the copy speed is obtained by dividing a minute by 784.14 ms, that is, 76.52 cpm (copy per minute). Likewise, in the case Z2, the sheet transfer cycle Z2a is 796.23 ms per a sheet and therefore the copy speed is 75.36 cpm. In the case Z3, the sheet transfer cycle Z3a is 812.60 ms per a sheet and therefore the copy speed is 73.84 cpm. In the case Z4, the sheet transfer cycle Z4a, the copy speed is 72.63 cpm. From the calculations above, it should be understood in both the initial condition and the after-predetermined-time-use condition that the greater the load against the sheet transfer, the lesser the copy speed.
Further, based on the above-mentioned time differences Z1b-Z4b, corresponding distances from the trailing edge of a recording sheet 2 to the next recording sheet 2 at the registration roller 11 in the continuous sheet feeding mode in the cases Z1, Z2, Z3, and Z4 are calculated in the following manner. In the case Z1, the time difference Z1b is 281.64 ms and therefore the distance is obtained by multiplying the time difference by the initial linear speed of the registration roller 11, that is, 0.28164 s multiplied by 400 mm/s which is equal to 112.66. Likewise, in the case Z2, the time difference Z2b is 293.73 ms and therefore the distance is 0.29373 s multiplied by 400 mm/s which is equal to 117.49 mm. In the case Z3, the time difference Z3a is 309.34 ms and therefore the distance is 0.30934 s multiplied by 399.4 mm/s which is equal to 123.55 mm. In the case Z4, the time difference Z4a is 322.82 ms and therefore the distance is 0.32282 s multiplied by 399.4 mm/s which is equal to 128.93 mm. From the calculations above, it should be understood in both the initial condition and the after-predetermined-time-use condition that the greater the load against the sheet transfer, the lesser the copy speed.
As such, the distance between the adjacent recording sheets in the continuous sheet feeding mode, which are worthless for the print operation, is growing. The sheet transfer linear speed after the registration roller 11 is predetermined as 400 mm/s in the initial condition and is reduced to 399.4 mm/s in the after-predetermined-time-use condition. That is, a difference between the sheet transfer linear speeds in the above-mentioned conditions is relatively small. Therefore, it should be understood that the growing difference between the adjacent recording sheets after the registration roller 11 is a major factor that adversely affects the printing productivity.
This patent specification describes a novel sheet transferring apparatus for use in an image forming apparatus. In one example, this novel sheet transferring apparatus includes a sheet transferring mechanism and a controller. The sheet transferring mechanism is arranged and configured to transfer a recording sheet at a transfer speed to an image forming mechanism in the image forming apparatus. The controller is arranged and configured to determine the transfer speed based on a transfer speed used for an immediately previous recording sheet.
The sheet transferring mechanism may include a transfer roller and at least two sensors. The two sensors are arranged and configured to detect a recording sheet being transferred. The two sensors are mounted with a predetermined distance from each other.
The controller may determine the transfer speed using an equation;
VR(n)={VR(nxe2x88x921)}2/V(nxe2x88x921),xe2x80x83xe2x80x83(5)
wherein n is an integer greater than 1, VR(n) represents a linear speed of the transfer roller when transferring an nth recording sheet, VR(nxe2x88x921) represents a linear speed of the transfer roller during a transfer of an (nxe2x88x921)th recording sheet, and V(nxe2x88x921) represents a moving speed of the (nxe2x88x921)th recording sheet. When the n is equal to 1, the linear speed VR(1) is set to a predetermined value.
The controller may apply a correction tolerance of xc2x15% to the equation (5) so that the transfer roller is driven at the linear speed R(n) within a range of;
[{VR(nxe2x88x921)}2/V(nxe2x88x921)]xc3x970.95xe2x89xa6R(n)xe2x89xa6[{VR(nxe2x88x921)}2/V(nxe2x88x921)]xc3x971.05.xe2x80x83xe2x80x83(6)
The controller may determine the transfer speed using an equation;
VR(n)=[{VR(nxe2x88x921)}2xc3x97T(nxe2x88x921)]/L,xe2x80x83xe2x80x83(7)
wherein n is an integer greater than 1, VR(n) represents a linear speed of the transfer roller when transferring an nth recording sheet, VR(nxe2x88x921) represents a linear speed of the transfer roller during a transfer of an (nxe2x88x921)th recording sheet, L represents the predetermined distance, and T(nxe2x88x921) represents a time period in which the (nxe2x88x921)th recording sheet is moved the predetermined distance. The n is equal to 1 the linear speed VR(1) is set to the predetermined value.
This patent specification further describes a novel image forming apparatus. In one example, this novel image forming apparatus includes an image forming mechanism, a sheet transferring mechanism, and a controller. The image forming mechanism is arranged and configured to form a visible image on a recording sheet. The sheet transferring mechanism is arranged and configured to transfer the recording sheet at a transfer speed to the image forming mechanism. The controller is arranged and configured to control a number of revolutions of a motor for driving the transfer roller to determine said transfer speed based on a transfer speed used for an immediately previous recording sheet.
This patent specification further describes a novel image forming system. In one example, this novel image forming includes an image forming apparatus and an operation apparatus. The image forming apparatus includes an image forming mechanism, a sheet transferring mechanism, and a controller. The image forming mechanism is arranged and configured to form a visible image on a recording sheet. The sheet transferring mechanism is arranged and configured to transfer the recording sheet at a transfer speed to the image forming mechanism. The controller is arranged and configured to determine the transfer speed based on a transfer speed used for an immediately previous recording sheet. The operation apparatus includes a display for indicating a warning that the sheet transfer mechanism is in a condition asking for an inspection in accordance with an instruction from the image forming apparatus when the transfer speed is varied out of predetermined limits.